Yes - it's strange. Even using iCab you've got to transfer the files you want to upload into iCab's local file space. i.e. iCab (and no other app, either) can directly access the file space of another app. So, for iCab to be able to upload a file to a website, it needs to be in iCab's local filespace. You'll have to use either the 'Open In' facility in iOS or transfer the files into iCab from your PC or Mac.
Tim